This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved alkaline flooding operations involving the injection of aqueous solutions of foam-forming surfactants and gases, or pre-formed foams, either ahead of or behind conventional alkaline slugs.
In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
The oil-water interfacial tension may be reduced, either by injecting aqueous surfactant solutions into the oil formation, or by forming surfactants in situ. In this regard, crude petroleum is known to contain varying amounts of acid components that upon reaction with alkaline agents form soaps (natural surfactants) in situ that reduce the interfacial tension between the crude petroleum and water. Accordingly, the use of alkaline waterflooding, to form such natural surfactants in situ as a recovery mechanism, has been extensively investigated.
Alkaline flooding improves oil recovery by one or more complex mechanisms, including interfacial tension reduction, emulsification of oil, and alteration of reservoir rock wettability. Interfacial tension reduction increases microscopic oil displacement by reducing the capillary forces which trap oil droplets in reservoir rocks. Emulsification is believed to promote increased oil recovery by entrainment of emulsified oil in carrier waters or by formation of viscous emulsions which tend to be entrapped and divert water to improve volumetric sweep efficiency. Wettability alteration produces changes in oil-water relative permeability characteristics which frequently results in lower residual oil saturations from waterflooding. These mechanisms, which operate to different extents in different reservoirs, are discussed in a review paper by E. H. Meyer et al entitled "Alkaline Waterflooding - Its Theory, Application, and Status", which was published in the Proceedings of the Second European Symposium on Enhanced Oil Recovery, Paris, Nov. 8-10, 1982.
When an oil reservoir is subjected to an alkaline flooding procedure, due to poor mobility or profile control substantial amounts of oil remain behind in the oil formation. Poor mobility control causes the alkaline flood to finger through more viscous oil and around the less permeable relatively oil-rich zones into the formation zones of high permeability and low oil saturation. To improve mobility control and sweep efficiency, expensive water soluble polymers such as xanthan gums or polyacrylamides have been used.
U.S. Pat. No. 3,977,470 to Chang and U.S. Pat. No. 4,004,638 to Burdyn et al discloses recovery of oil from subterranean reservoirs employing an alkaline agent and a sulfonate surfactant.
U.S. Pat. No. 4,773,484 to Chang et al discloses an oil recovery process in which an aqueous solution containing a surfactant is injected with or followed by a gas into a formation to provide enhanced mobility control.
The present invention provides a process for recovering oil from a formation by alkaline flooding with improved mobility and profile control using generally low-cost foam forming injectants that substantially plug the relatively high permeability zones of the formation resulting in increased total oil recovery.